Currently there are four major types of traffic or roadway sensors being employed to monitor vehicle traffic i.e., volume, speed, classification and weight-in-motion. The most common, the road tube, which generates an air pulse, is used for classification, volume and speed studies. The inductive loop is used for volume, speed and length classification studies. The capacitance mat is used for volume and weight-in-motion studies. Piezoelectric rod is used for volume, speed, classification and weight-in-motion studies. These sensors, and others, such as resistance strips, are used in conjunction with electronic traffic counters which establishes the monitoring parameters and stores results for generating traffic engineering reports.
A paper entitled "A Review of Current Traffic Sensor Technology" by the inventor hereof sets out and details the general constructions, methods of installation and advantages and disadvantages of these various systems and is incorporated herein by reference.
Roadway sensors which use the piezoelectric effect are known in the art as, for example, see Robert U.S. Pat. No. 4,383,239 wherein a piezoelectric sensor is isolated from roadway pressure waves by means of a system of roadway channel embedments. Surface mounting of switch plates or coaxial sensing cables to produce an electrical effect when subjected to pressure is disclosed in Myers U.S. Pat. No. 3,911,390. In the Myers patent, there is disclosed a multilane coaxial cable sensor in which individual sensor segments corresponding to lanes are connected to a recording unit. The traffic sensor is contained in a sealed envelope. A pair of spaced metallic plates are positioned in the envelope along the length thereof for nailing the sensor to the roadway. As an alternative, the sensor could be secured to the roadway by an adhesive material. Use of adhesive to secure roadway sensor switches to the pavement is also shown in Paver U.S. Pat. No. 2,067,336 wherein a rubber cement carried in a cloth tape secures a switch tube to the pavement. Adhesive tape has also been used for this purpose. The invention is applicable to all these various kind of roadway sensors where movement of the sensor caused by vehicle air turbulence can cause extraneous and/or ambiguous output signals.
The object of the present invention is to provide an improved roadway sensor, and method of attachment to the pavement of a roadway, particularly a roadway sensor fitted or equiped with a linear weight roadway sensor which is sufficient to itself maintain the sensor on road, and more particularly, to a flat roadway sensor which is fitted with a distributed linear weight of sufficient weight per unit length to maintain the flat sensor on the roadway at selected roadway speeds. For a high speed roadway where heavy trucks and the like travel at 75 mph a relatively heavy distributed weight is required, greater than one pound per foot and for speeds where the air flow effects are lower, a lower distributed weight is used.
A further object of the invention is to provide a roadway sensor, where if broken, (by being snagged by a dragging muffler, for example) is safer than prior art roadway sensors. A further object of the invention is to provide a safe method of installing roadway sensors. In a preferred embodiment of the invention, a roadway sensor is constructed of a thin flat piezo film strip having a system of sensing electrode patterns metallized on opposing surfaces thereof and signal conductors connected to the electrode patterns sandwiched or enveloped between two elastomeric strips with a flat linear weighting member embedded in or on one of the elastomeric strips so as to maintain the sensor on the roadway despite having loaded truck-trailer travel at high speeds generating a trailing air turbulence having the effect of sweeping the roadway. The weight member is uniformly distributed along the roadway portions of the sensor strip to maintain the sensor on the roadway and substantially immune to air effects generated by vehicular traffic on the roadway, a loaded truck-trailer traveling at high speeds, a flat malliable metal such as lead, strip having a weight greater than one pound per linear foot is required. For example, a lower speed and/or smaller lighter vehicles (which cause less air flow effects), a lower distributed weight can be used for example 1/2 pound per linear foot of sensor.
The invention provides a low cost, rugged and roadway sensor which is compact, easy to install and can be safely installed in a few minutes by one person without stopping traffic. It has a low profile providing multilane output signals with one sensor. Moreover, it is portable and reuseable. It is capable of wide temperature range of operation and has reliable consistent output pulses. It can generate axle pulses and provide voltage signals with no external power supply being required. Moreover, if it is broken, it is safer than previous roadway sensors because the distributed weight in its preferred form holds the portions to the roadway so the sensor does not flap around with air currents caused by vehicles on the roadway.
While the invention is applicable to all forms of road sensors fixed to a roadway surface, in a preferred embodiment, piezoelectric film such as PVDF referred to above, is provided with separate discrete electrodes in predetermined metallized patterns for each lane of the roadway. An insulating coating or film is applied to the film and electrodes and an conductive electrostatic shield is applied or formed over the insulating coating. In the preferred embodiment, this piezoelectric film is then laminated and sealed between two elastomeric or rubber strips to form a sensor envelope. In a specific embodiment, a high weight linear weighting strip, such as 13/4 inch wide, 1/8 inch thick lead is affixed to or incorporated into the sensor envelope or otherwise incorporated in the construction to maintain the sensor in contact with the roadway along the length thereof so that only compressive loading caused by vehicle wheels cause voltage signals to be generated in the electrodes on the piezoelectric film.
Moreover, static electricity generated by movement of a body in air, for example, which can create false signals and/or ambiguous signals, can be discharged into conductive elements, which can constitute at least a part of the high weight linear weighting strip and serve as an electrostatic shield which is incorporated in the sensor.
In one embodiment, the conductive electrostatic shield is a foil of mallable metal such as lead.
As disclosed in my above-identified application, the manner of installation of the sensor to the roadway is critical to its operation. At high speeds, such as on interstate highways, vehicular traffic, heavy trucks, for example, can create air flow and negative pressure effects of sufficient force on the roadway sensor to lift and move it and create ambiguous signals. This form of error or ambigupis signal can be generated in other varieties of roadway sensors. The uniform adherence of the flat envelope to the roadway aids in avoiding stretching along the length of the piezoelectric film. In one preferred embodiment, a PVDF piezoelectric film strip with metallized electrode patterns and signal conductors connected thereto is laminated between two pieces of flat roadway grade rubber or plastic strips with a linear weight member in the lower rubber strip. For example, a 13/4 inch wide, 1/8 inch thick lead strip about 28 feet long weighs about 32 pounds. A thin piezoelectric film strip of the type disclosed in my above application for a dual lane roadway is preferred. For lower speed highways, a lower weight can be used. For example, for 35 mph a lead strip 1/16 inch thick and 13/4 inch wide (greater than one-half pound per linear foot) could be used. Steel lead shot sand and liquids could also be used.
Electrical connections to the metallizations can be made in a number of ways, but in the preferred embodiment, separate electrical connections are made to each lane metallization and discrete wires or conductors carry the signal voltages which are generated due to compression stressing of the piezoelectric film. These discrete wires or conductors can be fine multistrand insulated wires individually connected to each metallization or conductors printed on a common non-conductive, inert substrate which is discrete or non-piezoelectric with respect to the piezoelectric film. For single lane roadways, the piezoelectric film strip can be made wider and the signal conductor leads printed on the film with the metallization of the sensor electrodes.
The pattern of electrodes for the piezoelectric film can be designed to detect the number of tires and axles for a given vehicle. A particularly useful application of the invention is in connection with intersection data capture systems in which multiple lane roadways meet at intersections wherein vehicles traversing the different lanes can turn in different directions at the intersection and it is desired to know the various turnings and vehicle traffic in the different roadways of the multilane highways.
In the preferred embodiment, the piezoelectric film strip and its electrode metallizations have an insulative conformal coating thereon to prevent moisture or other material from adversely affecting operation. The electrostatic shield can be a conductive coating applied at least to the upper external surfaces of the conformed coating, and/or a separate or discrete conductive grid sealed in the envelope and/or as conductive material, such as carbon granules embedded in the rubber or plastic envelope elements. As noted above, in one embodiment the electrostatic shield is a strip of lead foil weight to cause an adherence of the sensor to the roadway when heavy vehicles, such as trucks, create trailing turbulences and air currents tending to lift and drag the sensor. The weight distributed along the roadway portion of the sensor prevents this and thus prevents stretching along the length (axial stretch) of the sensor material. In a further embodiment, the electrostatic shield is a conductive epoxy which eliminates voids and secures the upper outer roadway rubber layer of the outer protective envelope to the insulating conformal coating. Where the electrostatic shield is within the envelope it is electrically connected by a separate conductor to earth ground, as distinct from the electrical ground or return of the piezoelectric sensor elements.
In a further embodiment, the piezoelectric film sensor is positioned over a flat lead strip and a latex layer is between the lead layer and the piezoelectric film. This layered assembly is enclosed in an envelope.